The invention relates to a device for the generation of electron beams in a vacuum chamber, in which a massive cathode with a filament anode positioned above, is arranged, below the massive cathode an anode with an aperture is positioned and below the anode with the aperture magnet arrangements are provided for focusing and/or deflecting an electron beam emitted from the massive cathode and accelerated by the aperture anode and directing it to a processing location.
Devices of the type mentioned above are known as so-called electron guns. These are generators of electron beams with a high electric power and a high pressure-de coupling between a chamber for the generation of beams and a processing chamber by stream resistances arranged in the electron gun.
Within such electron guns a massive cathode is provided. Above the massive cathode a spiral heating filament is located which is denoted as a wire cathode. Between the wire cathode and the massive cathode an auxiliary voltage of about 1000 V is applied ( a so-called shock potential). As a result the electrons emitted by the wire cathode move to the massive cathode, impinge there and heat it. The massive cathode thereupon itself emits electrons which are accelerated by an anode arranged below the massive cathode and leave as an electron beam the electrode arrangement through the aperture of the aperture anode. This electron beam is being focused by magnet arrangements and can be deflected by additional magnet arrangements. The electron beam impinges on a workpiece and heats it with a high efficiency.
Such electron guns are used in vacuum apparatus for heating, melting, and vaporizing when powers of about 50 kilowatt to several megawatt are required. These installations are manufactured with different power stages so that the required power can be reached. The individual matching of the power is done by regulating the power of each electron gun. Basically there are known three ways to accomplish this:
A first possibility is to vary the electron stream by changing the temperature of the cathode. A disadvantage of this is that on the one hand ,because of the great mass of the massive cathode, this change is relatively sluggish. On the other hand the diameter of the beam is increased, leading to a power loss of the beam on the way to the processing location because of high scattering losses. In addition only a small control range of the power in a ratio 1:3 can be achieved.
A second possibility for varying the power is by changing the accelerating voltage between the aperture anode and the massive cathode. The disadvantage of this is that when reducing the accelerating voltage the energy of the particles is reduced leading to a so-called weak beam. This beam is associated with high losses. A further disadvantage is that at such power changes the magnetic fields of the focusing lenses and the beam deflection of the electron beam and eventually also the magnetic fields of additional deflection fields have to be readjusted into the processing space.
A third possibility is to vary the field intensity between cathode and anode by adjusting the position of the aperture anode. Here a so-called position-regulated anode is known. This, for instance is described in the DD-economical patent 134 160. Hereby the position of the aperture anode in relation to the fixed massive cathode is varied. The apparent disadvantage of this is that also only a relatively small control range of the power in a ratio of 7:1 can be achieved. As a rule, however, a considerably greater control range is required. The control range is limited by the adjusting devices, which, because of the aperture in the middle of the aperture anode, can not engage at he center. For this reason the beam voltage has additionally to be varied so that the disadvantages of the second possibility show up.